1. Field of the Invention
The present invention relates to a surface-emitting device and a liquid crystal display device, and more particularly, to the configuration of a surface-emitting device that serves to utilize, as illumination light, more of the light emitted from a light source.
2. Description of the Related Art
Hitherto, in reflective liquid crystal display devices that produce display with ambient light used as a light source, the visibility of the display extremely decreases in an environment in which sufficient ambient light cannot be obtained, for example, when used in a dark place, because the luminance depends on the amount of ambient light. Accordingly, a liquid crystal display device has been proposed in which a front light (surface-emitting device) is placed in front of a reflective liquid crystal display unit (liquid crystal display element) so that it is used as an auxiliary light source. The liquid crystal display device having the front light operates as a normal reflective liquid crystal display device in an environment in which sufficient ambient light can be obtained, for example, outdoors in the daytime, and illuminates the front light as the light source, as necessary. An example of the configuration of such a front light will be described below with reference to FIGS. 11 to 13.
A front light 110 shown in FIG. 11 includes a flat light guide panel 112 formed by injection-molding a transparent acrylic resin or the like, and a plurality of (two in the figure) light sources 111 disposed at an end face 112a of the light guide panel 112. The lower surface of the light guide panel 112 in the figure serves as an emergent surface from which light for illuminating a liquid crystal display unit is emitted, and a surface (the upper surface of the light guide panel 112) on the opposite side of the emergent surface serves as a prism surface 112c on which projections 114 shaped like a wedge in profile are continuously arranged in parallel so as to change the direction of light propagating inside the light guide panel 112. The light sources 111 are point light sources such as white LEDs (Light Emitting Diodes) or organic EL (Electro Luminescence) elements, and are arranged so as to emit light toward the end face 112a of the light guide panel 112.
In the front light 110 having the above configuration shown in FIG. 11, light emitted from the light sources 111 is introduced into the light guide panel 112 through the end face 112a, and the light propagating therein is reflected by the prism surface 112c so as to change the propagating direction, and is emitted from the emergent surface (lower surface) of the light guide panel 112. The emitted light illuminates a liquid crystal display unit or the like placed on the back side of the front light 110.
However, since the front light 110 has a structure in which the point light sources 111 are placed at the end face 112a of the light guide panel 112, the intensity of the light introduced into the light guide panel 112 is inevitably nonuniform, and as a result, light emitted from the emergent surface is also nonuniform. Accordingly, in order to increase the uniformity of the emitted light, a front light 120 is in practical use in which a light guide bar 113 is provided between a light guide panel 112 and light sources 115, as shown in FIG. 12.
In this front light 120, as shown in FIG. 12, the light guide bar 113 is placed along an end face of the light guide panel 112, and the light sources 115 formed of an LED serving as a light-emitting element are placed at both ends in the longitudinal direction of the light guide bar 113. An outer side face (a side face on the opposite side of the light guide panel 112) 113a of the light guide bar 113 has a prismatic shape (not shown) that can reflect light propagating inside the light guide bar 113 so as to change the propagating direction.
Therefore, in the front light 120, light emitted from the light sources 115 is introduced into the light guide bar 113 through both end faces thereof, is caused by the prism surface formed on the outer side face 113a of the light guide bar 113 to change the propagating direction, and is introduced into the light guide panel 112 from the end face.
Since the front light 120 has the light guide bar 113 in this way, light is introduced from the entire connecting surface between the light guide panel 112 and the light guide bar 113 into the light guide panel 112, and this improves the uniformity of the light emitted from the emergent surface of the light guide panel 112.
While the distribution of emergent light is relatively uniform in the front light 120 having the above configuration, the luminance necessary to illuminate the liquid crystal display unit is insufficient. Furthermore, light introduced into the light guide panel 112 directly reaches the viewer from the surface (upper surface) of the light guide panel 112, and this causes a phenomenon in which the surface of the light guide panel 112 looks white (whitening), and reduces visibility. In order to solve this problem, a front light having a configuration shown in FIG. 13 has been suggested. In this front light 130, a metal cover member 118 having an angular-U profile is mounted from the side of the light guide bar 113 of the front light 130 shown in FIG. 12. This cover member 118 includes a reflecting-surface covering portion 118a placed at a reflecting surface 112c of the light guide panel 112, a light-guide covering portion 118c placed outside the light guide bar 113, and an emergent-surface covering portion 118b placed at an emergent surface (lower surface) of the light guide panel 112. These covering portions prevent light propagating inside the light guide bar 113 and the light guide panel 112 from being emitted in the directions other than the light guide direction (the direction from the light guide bar 113 toward the light guide panel 112).
In the front light 130 having such a configuration, since a decrease in luminance caused by the light loss due to the light leakage in the directions other than the light guide direction can be reduced, illumination with higher luminance than in the front light 120 shown in FIG. 12 is possible. However, since the display quality has recently been improved for higher-definition and higher-contrast liquid crystal display devices, the front lights have been required to further enhance the luminance and to further improve the uniformity of emergent light, and the development of front lights that achieve a higher-luminance and more uniform illumination has been demanded.
Accordingly, one object of the present invention is to provide a surface-emitting device that achieves high utilization efficiency of a light source, high luminance, and high uniformity of emergent light.
Another object of the present invention is to provide a liquid crystal display device having a surface-emitting device that has the above superior characteristics and achieves high visibility.
In order to achieve the above objects, according to one aspect, the present invention provides a surface-emitting device including a light guide panel, a light guide placed along an end face of the light guide panel, and light sources placed at opposing ends of the light guide, wherein a reflecting surface of the light guide panel has irregularities for reflecting light propagating therein, an emergent surface of the light guide panel emits light reflected by the reflecting surface, a cover member including at least a reflecting-surface covering portion that covers an end of the light guide panel on the side of the reflecting surface, a light-guide covering portion that covers the light guide, and an emergent-surface covering portion that covers an end of the light guide panel on the side of the emergent surface is mounted so as to cover at least the light guide and the ends of the light guide panel, and the reflecting-surface covering portion protrudes longer in the light guide direction than the emergent-surface covering portion (in other words an end of the reflecting-surface covering portion is disposed more distal to the light guide than an end of the emergent-surface covering portion).
In the surface-emitting device of this invention, more light inside the light guide panel can propagate toward the emergent surface of the light guide panel by setting the length of the reflecting-surface covering portion of the cover member that covers the light guide and the ends of the light guide panel larger than that of the emergent-surface covering portion. Since this structure can prevent light from leaking from the reflecting surface of the light guide panel, the utilization efficiency of the light sources can be enhanced, and the luminance of the surface-emitting device can be increased. Furthermore, since the light leakage toward the reflecting surface of the light guide panel is limited, whitening can be effectively reduced.
Preferably, the reflecting-surface covering portion protrudes at least 0.5 mm farther than, and more preferably, at least 0.7 mm, in the light guide direction than the emergent-surface covering portion.
Since the amount of light propagating toward the emergent surface of the light guide panel can be further increased by thus setting the difference in length, the luminance of the surface-emitting device can be increased further. The present inventor has proven in examples provided herein that the above values increase the amount of luminescence by an appreciable amount.
A reflective layer made of a metal thin film may be formed on the inner surface of the cover member. Since this structure makes it possible to reflect and return light, which is introduced from the light guide or the light guide panel into the cover member, to the light guide or the light guide panel while limiting light loss, the utilization efficiency of the light sources can be enhanced.
According to another aspect, the present invention provides a surface-emitting device including a light guide panel, a light guide placed along an end face of the light guide panel, and light sources placed at opposing ends of the light guide, wherein a reflecting surface of the light guide panel has irregularities that reflect light propagating therein, an opposing emergent surface of the light guide panel emits light reflected by the reflecting surface, a reflective film made of a metal thin film that reflects light propagating inside the light guide and the light guide panel is formed on the light guide and an end of the light guide panel, and the length of the reflective film from the end face of the light guide panel at which the light guide is placed is larger on the side of the reflecting surface of the light guide panel than on the side of the emergent surface of the light guide panel.
In order to prevent light leakage from the light guide, the light sources, or the end of the light guide panel, the reflective film made of a metal thin film is formed on the surfaces of these members. This makes it possible to prevent the above-described light leakage, to limit the amount of light lost when being reflected by the surface of the reflective film, and to thereby efficiently utilize the light from the light sources.
An antireflection layer may be formed on one or opposing of the end face of the light guide panel and a side face of the light guide facing each other. This makes it possible to prevent light from being reflected and returned toward the light guide by the end face of the light guide panel when being introduced from the light guide into the light guide panel, thereby achieving a higher-luminance surface-emitting device. That is, when light is reflected by the end face of the light guide panel, the light returned to the light guide is reflected by the reflecting surface of the light guide and enters the light guide panel again. Since such light travels in a direction that is unintended in the design of the light guide, when it is emitted from the light guide panel, the uniformity of the amount of emergent light may be decreased, and it is preferable to eliminate as much of the light as possible. The above-described structure can eliminate such reflected light.
Preferably, a side face of the light guide facing the end face of the light guide panel serves as an emergent surface for applying light from the light sources onto the light guide panel, a side face on the opposite side of the emergent surface serves as a reflecting surface on which concave grooves, each having a pair of inclined faces for reflecting light propagating inside the light guide, are periodically formed at a predetermined pitch, and the angle formed between the two inclined faces constituting the groove is within the range of 105xc2x0 to 115xc2x0.
By determining the shape of the concave grooves, which have an influence on the uniformity of light emitted from the light guide, as described above, in order to improve the uniformity of light applied from the light guide onto the end face of the light guide panel, the light emitted from the light guide is efficiently supplied to the light guide panel, and the uniformity of light emitted from the light guide is improved, thereby increasing the amount of light emitted from the principal surface of the light guide panel and improving the uniformity of the light. Preferably, the depths of the concave grooves closer to the center of the light guide are larger than the depths of the concave grooves closer to the light sources. The distribution of the amount of emergent light in the longitudinal direction of the light guide is thereby made uniform.
Preferably, the angle formed between the two inclined faces constituting the concave groove is set to be within the range of 105xc2x0 to 115xc2x0. Such a range makes it possible to increase the amount of light emitted toward the light guide panel, to further enhance the utilization efficiency of the light sources, and to achieve a higher-luminance surface-emitting device. When the angle is less than 105xc2x0, light is not emitted in a desired direction, and as a result, the luminance decreases. When the angle exceeds 115xc2x0, the uniformity of the amount of emergent light cannot be maintained. Opposing cases are undesirable.
Preferably, the angle formed between the two inclined faces constituting the concave groove is set to be within the range of 108xc2x0 to 112xc2x0. By setting the angle within such a range, the amount of light emitted toward the light guide panel can be further increased, and the luminance of the surface-emitting device can be increased.
Preferably, the distance between the emergent surface and the reflecting surface of the light guide is set to be within the range of 3.2 mm to 5.0 mm. By setting the distance between the emergent surface and the reflecting surface within such a range, light introduced from the light guide into the light guide panel can be prevented from leaking from the principal surface of the light guide panel, and the luminance of the surface-emitting device can be thereby increased.
Preferably, the centers of light-emitting portions of the light sources are aligned with almost the center in the thickness direction of the light guide. This makes it possible to improve the uniformity of light emitted from the light guide, and the uniformity of the amount of light will not be impaired even when a large-area light guide panel is used.
According to a further aspect, the present invention provides a liquid crystal display device wherein any of the above-described surface-emitting devices is provided in front of the liquid crystal display unit. Since this allows the liquid crystal display unit to be uniformly illuminated with high luminance by the surface-emitting device, it is possible to provide a liquid crystal display device having high visibility.
Further objects, features, and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.